At one time, metal-based amalgams, then porcelain or other ceramic materials were used in a variety of remedial dental procedures. Now, synthetic composites are used as practical alternatives to these materials for such procedures. A composite is a polymer, otherwise referred to as a resin, which has at least one additive. An additive can be anything added to the polymer or resin to impart a desired property. The composite generally starts out as a paste or liquid and begins to harden when it is activated, either by adding a catalyst, adding water or another solvent, or photoactivation. Advantageously, synthetic composites provide an aesthetically more natural appearance versus porcelain or other ceramic materials.
Synthetic composites are typically made from complex mixtures of multiple components. Synthetic composites must be completely dissolvable in a fluid vehicle, yet remain flowable and viscous; undergo minimal thermal expansion during polymerization; be biocompatible with surrounding surfaces of tooth enamel and colloidal dentin; and, have aesthetic similarity to natural dentition in terms of color tone and polishable texture. Furthermore, the synthetic composite must have sufficient mechanical strength and elasticity to withstand ordinary compressive occlusive forces, without abnormal wearing and without causing abrasion to dentinal surfaces.
The different varieties of synthetic composites may be approximately divided into three main groups of products: synthetic resin-based dental composites, glass-based dental composites, and hybrid dental composites.
A synthetic resin-based dental composite typically comprises several monomers combined together. A monomer is a chemical that can be bound as part of a polymer. The synthetic resin-based dental composite includes other materials, such as silicate glass or processed ceramic that provides an essential durability to the composite. These materials may also be made from an inorganic material, consisting of a single type or mixed variety of particulate glass, quartz, or fused silica particles. Using differing types of inorganic materials, with differing diameter sizes or size mixtures, results in differing material characteristics.
Glass-based dental composites are made from a glass particles, such as powdered fluoroaluminosilicate, dissolved in an aqueous polyalkenoate acid. An acid/base reaction occurs spontaneously, causing precipitation of a metallic polyalkenoate, which subsequently solidifies gradually. The glass particles may be made from silicate, such as silicone dioxide or aluminum silicate, but may also include an intermixture of barium, borosilicate, alumina, aluminum/calcium, sodium fluoride, zirconium, or other inorganic compounds. Some of the earlier glass-based composites were formulated to contain primarily a mixture of acrylic acid and itaconic acid co-monomers. However, more recently such hybrid products are modified to include other polymerizable components, such as HEMA or BisGMA.
Hybrid composites are the third category of synthetic dental composites. Hybrid composites combine glass particles with one or more polymers. Hybrid composites may comprise water-soluble polymers other than polyalkenoate, such as hydroxyethyl methacrylate (HEMA) and other co-polymerizing methacrylate-modified polycarboxylic acids, which are catalyzed by photo activation. Other hybrid composites may be modified to include polymerizable tertiary amines, catalyzed by reaction with peroxides.
Synthetic dental composites are increasingly used more often for dental procedures, such as restoration and repair. Restoration and repair includes, for example, fillings, crowns, bridges, dentures, orthodontic appliances, cements, posts and ancillary parts for dental implants to name a few. Most common, synthetic dental composites are used for anterior Class III and Class V reconstructions, for smaller size Class I and Class II molar reconstructions, for color-matching of cosmetic veneers, and for cementing of crowns and overlays. Nonetheless certain disadvantages of these materials have been noted. For example, the trace amounts of unconverted monomers and/or catalyst that may remain within the composite and, if subsequently absorbed systemically in humans, may be potentially physiologically harmful.
Another major drawback associated with synthetic composites is that they tend to wear more rapidly, especially when placed in appositional contact with load-bearing dental surfaces, a deficiency that often limits the purposeful use of such materials primarily to repair of defects within anterior maxillary or readily visible mandibular surfaces.
Perhaps the most significant disadvantage associated with synthetic composites is that they have a comparatively lower resistance to fracture. Even relatively minor surface discontinuities within the composite, whether occurring from injurious trauma or occlusive stress, may progressively widen and expand, eventually resulting in partial or complete disintegration of the reconstruction or repair. This greater susceptibility to fracture is thought to be correlated with the dental reconstruction or repair.
Fracture susceptibility is also correlated with the proportional volume of the amount of synthetic composite required, or the lesser fraction of intact enamel and dentinal tooth material that remains available, prior to reconstruction or repair. It is well established from studies of the “cracked tooth syndrome” that once a damaging fracture has occurred, tooth loss may be almost inevitable, especially for carious teeth that have been previously filled. An improved synthetic composite having greater resistance to fracture would be significantly advantageous.
Synthetic composites having self-healing characteristics are known in the art, as illustrated for example in U.S. Pat. Nos. 6,518,330 and 6,858,659, describing self-repair of a polyester material containing unreacted amounts of cyclopentadiene (DCPD) monomer stored within a polyester matrix resin, as sequestered within polyoxymethyleneurea (PMU) microcapsules. From a fracturing mechanical stress sufficient to cause rupturing of one or more microcapsule, the monomer is reactively released. As the monomer contacts the polyester matrix, a polymerization occurs. The in situ polymerization occurs as a result of a ruthenium-based Grubbs catalyst or Schrock catalyst, which may be incorporated into the matrix. Alternatively, the catalyst may be stored within a fraction of separately prepared microcapsules, or may be contained within the same material comprising the microcapsule outer wall.
Although these patents disclose a composite having self-healing characteristics, there is still a demand for dental restorative composites having self-healing characteristics, or capability to autonomically resolve discontinuities occurring in the composite as well as methods of making such composites. The present invention satisfies this demand.